Make DataChannels Transferable and explain the implications

[ekr]

@martinthomson PTAL

[stefhak]

The actual transfer algorithm is not described. Look at e.g. https://html.spec.whatwg.org/multipage/scripting.html#transferCanvasProxy or https://html.spec.whatwg.org/multipage/comms.html#transferMessagePort (the transfer for ArrayBuffer also describes some error handling for the case when the ArrayBuffer was already neutered).

[alvestrand]

Tagging @annevk because I think the comment originated from him.

[annevk]

Yeah, you will need to define what transferring actually means (presumably you create a new instance within the other realm and move some internal slot) and how it affects the original object (presumably some operations will start failing). whatwg/html#444 has another example of an object that is being made transferable.

[stefhak]

This PR (and #317 ) needs work. If no one does the work it can't be part of 1.0.

[alvestrand]

Closing while preparing for 1.0. It can be reopened later.